Polymers containing thiocarbonylthio groups can be colored, the color depending on the absorption spectrum of the thiocarbonylthio chromophore. Such polymers can also be odorous or release an odor over time due to decomposition of the thiocarbonylthio groups. The presence of such color and odor can be disadvantageous. In some circumstances, it may also be necessary to deactivate thiocarbonylthio groups because of their reactivity or to transform them for use in subsequent processing into conventional applications, such as coating compositions.
For certain applications, it is desirable to have polymers possessing thiol functionality. These applications include the use of bis- or multithiols in the synthesis of condensation polymers such as polythiourethanes and polythioesters or polymers containing disulfide linkages. Polymeric thiols can also be used as transfer agents in free radical polymerization to form block or segmented copolymers.
Thiol functionality can also be used to form crosslinks in network polymers and microgels and to form conjugates to biological polymers, such as proteins.
Other applications of thiol-functional polymers relate to the property of thiols to complex metals, such as gold and cadmium. Thus, it is possible to use thiol-functional polymers to form nanoparticles encapsulating these metals. It is also possible to use such polymers as metal sequestering agents.
It is well known that thiocarbonylthio groups can be transformed into thiols by reaction with certain nucleophiles which include primary and secondary amines, ammonia, other thiols, and hydroxide. They can also be reduced to thiols with reducing agents such as sodium borohydride, lithium aluminum hydride or zinc in acetic acid. These and similar reactions of thiocarbonylthio groups in small molecules are reviewed by Kato and Ischida in Sulfur Reports, 1988, 8, 155 and by Mayer and Scheithauer in Houben-Weyl Methods of Organic Chemistry, volume E, p 891 (1985). Examples, of aminolysis of a thiocarbonylthio group from a polymer to leave thiol functionality are provided in Makromol. Chem. 1982, 182, 2383.
Polymers formed by RAFT polymerization contain thiocarbonylthio groups either at the chain ends or within the chain. In WO9801478A1, in which RAFT polymerization using thiocarbonylthio transfer agents is first disclosed by Le et al., it is also disclosed that polymers formed by RAFT polymerization are susceptible to this chemistry. The potential of this chemistry to cleave end groups and decolorize polymers and produce a polymer with a thiol end group is cited in by Chiefari et al. in Macromolecules 1998, 31, 5559. In Macromolecules 2000, 33, 244, it is demonstrated that the poly(methyl acrylate) prepared with a trithiocarbonate RAFT agent can be cleanly cleaved by treatment with ethylenediamine.
Application US20040171765A1 assigned to Kaneka Corporation claims treatment of a polymer with thiocarbonylthio chain ends with a compound selected from ammonia, hydrazine, primary amine compounds, and secondary amine compounds to form a polymer with thiol chain ends which is used as a precursor to a polythiourethane. The treatment agent was preferred to be low boiling (<100°), for example, ethylamine, or a hindered amine light stabilizer.
U.S. Pat. No. 6,794,486 assigned to Rhodia discloses the treatment of a polymer with a dithio group at the chain end with an amine not triethanolamine in an organic solvent to decolorize the polymer and remove end groups. The example amine provided is ammonium hydroxide. It is also suggested that many amine compounds can react with functionality in the polymers such as poly(acrylate esters) and poly(vinyl acetate) to give undesired side products. The process is not specifically directed to the synthesis of polymeric thiols and the end groups formed by removal of the dithio end group are not defined.
The by-products formed by cleaving a thiocarbonylthio end group to form a thiol end group by the processes described can themselves be colored or odorous. They may also interfere with subsequent processing steps or the intended application. They can also be difficult to separate from the polymer. Thus a new process is required which cleanly produces thiol groups from a starting polymer with thiocarbonylthio end groups and which provides byproducts which can be conveniently separated from the derived polymer.